1. Field of the Invention
The present invention relates to optical systems and optical apparatuses including the optical systems.
2. Description of the Related Art
To date, optical systems having a short lens length (optical length; length from a first lens adjacent to an object to an image plane) have been demanded for optical apparatuses such as digital still cameras, video cameras, and projectors.
In general, compact optical systems having a short lens length tend to suffer from a greater amount of aberration, in particular, a greater amount of longitudinal and lateral chromatic aberration, resulting in lower optical performance.
In particular, the amount of chromatic aberration is increased in optical systems having a shortened lens length.
Well-known optical systems include lens systems of a so-called negative-lead type having a lens unit with a negative refractive power disposed in the anterior position and a lens unit with a positive refractive power disposed in the posterior position. Herein, the anterior position of the optical systems refers to a position adjacent to an object in the case of optical imaging systems such as cameras, or a position adjacent to a screen in the case of optical projection systems such as liquid-crystal projectors (magnification side). Also, the posterior position of the optical systems refers to a position adjacent to an image-taking side in the case of the optical imaging systems, or a position adjacent to an original picture in the case of the optical projection systems (demagnification side).
Since optical systems of the negative-lead type have a lens unit with a negative refractive power in the anterior position, negative distortion can easily occur. In order to correct the aberration, materials having high refractive indices may be used for negative lenses in lens units having a negative refractive power.
However, glass having a high refractive index generally has a high dispersion, and easily causes a greater amount of negative lateral chromatic aberration.
In order to reduce the generation of chromatic aberration, materials having anomalous partial dispersion may be used, or diffractive optical elements may be disposed in optical paths.
In general, a lens having a positive refractive power, which includes a low-dispersion optical material with an anomalous partial dispersion such as fluorite, and a lens having a negative refractive power, which includes a high-dispersion optical material, are used for reducing the occurrence of chromatic aberration. Optical systems including such lenses have been discussed in, for example, Japanese Patent Laid-Open No. 2002-62478 and U.S. Pat. No. 6,404,561.
Moreover, liquid materials having a relatively high dispersion and a relatively high anomalous partial dispersion, and achromatic optical systems including such liquid materials are also known (U.S. Pat. No. 4,913,535).
When a material having an anomalous partial dispersion is used for correcting the chromatic aberration of an optical system, the number of lenses in the optical system tends to be increased, thereby increasing the optical length. Moreover, it is very difficult to machine anomalous partial dispersion glass such as fluorite.
Furthermore, the specific gravity of the material is relatively large compared with that of other low-dispersion glasses without anomalous partial dispersion, and thus the entire lens system tends to be increased in weight. For example, the specific gravity of fluorite is 3.18, and that of a glass known under the trade name of FK01 is 3.63.
In contrast, the specific gravities of trade name materials FK5 and BK7 having low anomalous partial dispersion are 2.46 and 2.52, respectively.
Surfaces of anomalous partial dispersion glasses are easily damaged, and anomalous partial dispersion glasses having large diameters can be easily cracked with sudden temperature changes. Thus, application of materials having anomalous partial dispersion to optical systems is limited.
Liquid materials as discussed in U.S. Pat. No. 4,913,535 require structures for hermetically containing the liquid materials, resulting in difficulties in manufacturing optical systems including such materials. Also, it is difficult to apply the liquid materials to optical systems due to changes in refractive indices and dispersion characteristics according to temperature.